normal baking, low poly to high poly. Sub surface corners not correct
I just checked your file and after baking everything looks pretty good. I can see that you added one more edgeloop to the high poly object and that seems to have fixed the issue. I only managed to replicate your image when the edgeloop was removed. When baking you still want to keep the objects as close as possible. There are some things that just aren't possible, like making a cube look like a sphere. Without that last edgeloop the cubes are just too different to get a good bake.